The present invention relates to a system for controlling a spindle motor and other motor or motors of a machine tool or the like for their synchronous operation, and more particularly to a system for controlling motors for synchronous operation which is suitable for use with a gear hobbing machine, a gear grinder, an engine lathe or the like in which two or more motors are required to rotate synchronously for gear cutting, gear grinding, thread cutting or the like.
Machine tools such as gear hobbing machines, engine lathes and the like cut gears and threads by rotating two or more motors including a spindle motor in synchronism. For example, it is necessary for gear cutting on a gear hobbing machine that a hob and a workpiece be rotated in complete synchronism. No high-precision gear cutting would be possible if the motors were rotated out of synchronism. More specifically, it is general practice for a hob shaft to make exactly one revolution while the workpiece is moving one pitch to cut a gear tooth on the workpiece. If the hob shaft and the workpiece rotate out of snychronism, then various cutting errors are caused which include a curved gear tooth, a varied gear pitch, and the like. For gear cutting on an engine lathe, it is necessary to bring the speeds of rotation of a workpiece on the spindle and a cutting tool into complete synchronism. If the workpiece and the cutting tool do not rotate synchronously, then gear teeth cannot be cut to a nicety. More specifically, the cutting tool moves one pitch while the workpiece makes one revolution. When the cutting tool moves out of synchronism with the rotation of the workpiece, the gear pitch is caused to vary thus resulting in a cutting error.
It has been customary in providing the synchronized motor operation to rotate a reference motor or spindle motor at a given commanded speed, attach a pulse coder to the reference motor for generating a pulse each time the motor rotates through a predetermined angle, and rotate another motor in synchronism with the pulse thus generated. More specifically, when the reference motor under the control of the commanded speed rotates at a speed of Vm.sub.1, the speed of the pulse is proportional to the motor speed Vm.sub.1, that is, kVm.sub.1. The speed Vm.sub.1 of rotation of the reference motor is detected, and the ratio of a speed Vm.sub.2 of rotation of the other motor to the speed Vm.sub.1 is determined. Data on the ratio Vm.sub.2 /Vm.sub.1 and the pulse having the pulse speed kVm.sub.1 are supplied to a pulse rate multiplier for multiplication. Then, the pulse rate multiplier produces a train of pulses having a pulse speed of kVm.sub.2, which is utilized to control the other motor to rotate in synchronism with the reference motor.
With the conventional system for controlling motors for synchronous operation, however, each motor is given a speed command, and hence only a speed control loop can be employed. This has led to the disadvantages in that the motor cannot readily restore its rotation at a commanded speed when its actual speed of rotation varies, and the speed of rotation of the spindle motor can easily change with a variation in the motor load. Furthermore, when the speed of rotation of the spindle motor varies due to a load variation or the difference in load between rough machining and final finishing of a workpiece, the amount of delay of operation of each motor tends to change as the speed command for the other motor is derived from the speed of rotation of the spindle motor, thereby lowering a cutting accuracy to a large degree.